The mechanism of the stearyl coenzyme A desaturase of hen liver microsomes will be investigated. The desaturase is responsible for the synthesis of unsaturated fatty acids and is essential to the biosynthesis and regulation of the lipid component of membranes. In addition to this key metabolic role the desaturase is itself a component of the endoplasmic reticulum and will be used as a "marker" enzyme in studies of membrane reassembly. Previous work has demonstrated that the desaturase is multicomponent in nature and that it can be resolved into NADH-cytochrome b5 reductase, cytochrome b5, a large molecular weight protein fraction and a lipid component. Recombination of these four fractions regenerates desaturase activity. These fractions will be further purified and investigated to elucidate their role in desaturation. If the "native" desaturase can be reassembled from its component parts in vitro this will support the hypothesis that such a complex, and probably the whole membrane, is biosynthesized in vivo by a spontaneous assembly of pre-formed lipid and protein components. The interactions between the lipid and protein components of the desaturase will be investigated by following the changes in physical and enzymatic properties of the individual protein components as the desaturase complex is reassembled. Such studies may indicate the factors controlling this in vitro and hence in vivo assembly. This research project will, therefore, investigate the factors controlling the synthesis of the lipid component of membranes as well as providing a model for the biosynthesis of the whole membrane. Similar procedures could be used with neoplastic membranes to ascertain whether their aberration is due to lack of control of the enzymes involved in synthesis of membrane components, for example, the lipid synthesizing enzymes, or to lack of control in their assembly.